1. Field of the Invention
This invention relates to a silver halide photographic emulsion and more particularly to a silver halide photographic emulsion chemically sensitized by a sulfur compound. The invention also relates to photographic materials containing such silver halide emulsions.
2. Description of the Prior Art
Various chemical sensitizations for increasing the light sensitivity of silver halide photographic emulsions are known. One of the typical methods is known as a sulfur sensitization wherein the light sensitivity of a silver halide photographic emulsion is increased by adding thereto a very small amount of sulfur or a sulfur compound to form silver sulfide as disclosed, for example, in U.S. Pat. Nos. 2,410,689 and 3,501,313, West German Pat. No. 1,422,869 and Japanese Patent Publication No. 20533/74.
Also, a method of increasing the light sensitivity of silver halide photographic emulsions by adding thereto a suitable reducing agent or gold compound is known as a reduction sensitization method or gold sensitization method respectively as disclosed, for example, in U.S. Pat. Nos. 2,399,083 and 3,297,446.
Furthermore, it is known to further increase the light sensitivity of silver halide emulsions by a combination of these sensitization methods as described in T. H. James, The Theory of the Photographic Process, 4th Edition, pp. 149 to 160 (Macmillan Co., 1977).
In these convetional sensitization methods, as the amount of sensitizer is increased to obtain higher light sensitivity, the silver halide photographic emulsions form fog greatly and it is difficult to control the formation of fog even by using antifoggants or stabilizers. Also, the silver halide photographic emulsions sensitized by such conventional sensitization methods have a disadvantage that when photographic films prepared by coating such silver halide emulsions are stored under high temperature and high humidity conditions, the photographic characteristics change greatly.
Attempts to improve the light sensitivity of silver halide emulsions by prolonging the chemical ripening period or increasing the ripening temperature are accompanied by an increase in the formation of fog and it is difficult to attain the desired purpose.